Vapor generator



Oct. 14, 1969 W. P. GORZEGNO ET AL VAPOR GENERATOR Filed OCT.. 11, 1967 Ric/1am' H Thomas ATTORNEY 'United States Patent Q 3,472,208 `VAPOR GENERATOR Walter P. Gorzegno, Florham Park, and Albert J. Zipay, Clifton, NJ., assignors to Foster Wheeler Corporation, Livingston, NJ., a corporation of New York Filed Oct. 11, 1967, Ser. No. 678,477 Int. Cl. F22b 35/10; F01k 7/22 U.S. Cl. 122-406 7 Claims ABSTRACT F THE DISCLOSURE This invention relates to a once-through vapor `generator of the forced ow type, including a method of startup and in particular to an arrangement of heating surfaces in combination with a start-up bypass system by which shock to the turbine is avoided during the start-up period.

It is well known in connection with start-up of a oncethrough vapor generator provide a bypass system around the turbines which protects the furnace circuits and turbines during the start-up period, passing a minimum ow through the furnace circuits until the iiow is heated sufficiently for admission to the turbines. Associated with the bypass system is a stop valve in the main flow path which when closed causes the How to enter the bypass system. The bypass system usually includes a :Hash tank in which the flow is separated into separate steam and water streams, and a return line to pass at least a portion of the separated steam to the turbine for early warming and rolling. The system also conventionally includes a plurality of valves by which the bypass system can be isolated from the main flow path for normal operation ow directly through the generator to the turbine.

When turbine demand reaches a point in the start-up period requiring pressures exceeding the design pressure of the flash tank, it is necessary to take the ash tank and bypass system out of operation, since any further increase in flash tank pressure could not be tolerated. A difficulty frequently experienced is a drop in enthalpy at the turbine inlet during this change-over, since the shutdown of the bypass ow results in a shift in the composition of the ow, downstream of the bypass system connections, from a saturated vapor (from the flash tank) to the lower enthalpy water-vapor mixture (from the main flow line). Usually, the time of change-over is delayed as long as possible, for instance, up to 15% load, to bring the enthalpy of the main line flow up to as close to that of saturated vapor as possible, but despite this, the drop in enthalpy with a resulting drop in temperature is experienced with most units.

An increase in firing rate only to remedy the problem is not an available solution, since the finishing superheater which, because of the location of the bypass, may have a minimum iiow of cooling fluid through it during periods of start-up insufficient to prevent overheat and possible burn-out. Usually the bypass system connections are upstream of the finishing superheater. An alternative can be used, namely orientation of the surfaces; for instance, location of surface upstream of the bypass system connec- 3,472,208 Patented Oct. 14, 1969 ACC tions, such as a primary superheater, in a high temperature or radiant heat transfer area of the generator. This may result in an increase in the enthalpy level of the iiow upstream of the bypass system connections closer to saturration at the time of change-over, but :it freezes the generator design to an arrangement of surfaces which may not be preferred.

It was described in co-pending application, Ser. No. 281,452, that certain advantages were accrued by positioning the conventional pressure reducing valve used in a start-up system in the main flow path upstream of a heating surface, removed from the bypass line by the heating surface, to obtain a substantial increase in heat transfer to the heating fluid during start-up. The pressure reducing valve maintained a high pressure in the furnace circuitry to protect the furnace circuitry from steam-water distribution problems, but in the heating surface between the pressure reducing valve and the bypass line, a much lower pressure. At the much lower pressure, the heat transfer could be as much as seven to eight times as great. The object of the invention was to reduce the start-up time.

However, it was also discovered in accordance with the present invention that the teachings of Ser. No. 281,452 could advantageously be applied towards achieving an enthalpy balance of bypass flow and main line flow at the time of change-over, and other advantages. In essence, the heat transfer into the primary circuit is maximized by locating the pressure reducing valve upstream thereof, in accordance with the teachings of Ser. No. 281,452. This arrangement permits positioning the primary circuit in a low temperature convection heating zone of the furnace, since even as so disposed the higher heat transfer rate assures that the enthalpy level of saturated vapor will be reached upstream of the bypass system connections prior to change-over to main line flow. This in turn frees the higher temperature heating zones for the finishing superheater permitting the latter to be disposed at least in part in a radiant portion of the furnace for better control of final steam temperature. Also, by virtue of the high heat, pick-up in the primary superheater, the generator can be fired at a lower safe firing rate during start-up, compatible with the finishing superheater load flow to avoid overheating of this surface, and still achieve a saturated steam condition upstream of the bypass system at `the time of change-over.

The invention and advantages thereof will become apparent upon further consideration of the specification, with reference to the accompanying drawing, in which the ligure is a combined section view and ilow diagram illustrating the invention.

Referring to the drawing, there is illustrated the convection area or rear portion 10 of a vapor generator wherein gases flow from a radiant portion of the generator, area 12, across screen tubes 14 which more or less mark the separation between the radiant portion of the generator and the convection area. From the screen tubes 14, the gases flow in an approximately L-shaped passage generally designated item 16, comprising a first horizontal area 18 and a downwardly extending area 20 divided into parallel passes 22 and 24. The gases are admitted into the second parallel pass 24 through screen tubes 26 between the passes.

The pertinent surfaces within the portion of the generator shown in the drawing are a pendent U-shaped superheater section 28, positioned adjacent the exit of the radiant area of the boiler immediately in front of the screen tubes 14, a U-shaped finishing superheater section 30 disposed in the horizontal flue gas flow area of the convection part of the boiler immediately behind the front screen tubes 14, and a primary superheater bank 32 positioned in the second of the downwardly extending parallel passes, item 24. Also shown is reheater bank 34 positioned at a lower elevation in the first of the parallel passes 22. Economizer 36 is also disposed in the parallel passes downstream, with respect to the gas flow, of the reheater and primary superheater banks.

The routing of the high pressure fluid flow circuitry is through the economizer 36, through tubes defining the furnace or radiant enclosure of the generator (portions of these tubes are shown and are designated with the numeral 38), through the screen tubes 14, through the enclosure Walls for the convection portion of the boiler (terminating in upper headers 40), and through roof tubes 42 via conduits 44 between headers 4t) and the inlet headers for the roof tubes. The roof tubes terminate in outlet header 46 and conduits from the header 46, items 48, transmit the fluid to inlet header 50 for the primary superheater. From outlet header 52 for the latter, the fluid is transmitted via conduit 54 to inlet header 56 for the pendent superheater 28. Header 58 serves as the outlet for the pendent superheader and inlet for the finishing superheater 30. The numeral 60 designates Athe outlet header for the finishing superheater, the fluid flowing therefrom via line 62 to the high pressure turbine `64, reheater 34 (for the purpose of clarity, a separate box also designated with the numeral 34 is used in the schematic ow diagram portion of the figure for the reheater), and the low pressure turbine 66 in that order. The condenser 68 receives the flow from the low pressure turbine, from which the flow is recycled through feed heaters for regenerative heating (generally designated with the numeral 70), via feed pump 72 to the economizer 36.

During start-up of the generator, a bypass circuit 80 is provided designed to divert the flow from the turbines to prevent the transmission of water or a water-vapor mixture to the high pressure turbine. The circuit 80 comprises a take-off line 82 connected at point 83 downstream (with respect to the flow of the Working fluid) of the primary superheater 32. Also part of the bypass line is flash tank 84, and return line 86 connected between a vapor section of the flash tank and a point 88 upstream of the pendent superheater 28, but downstream of the point of connection of the take-off line `82. Lines 90 and 92 serve the purpose of transmitting vapor and liquid flows from the flash tank to `the high pressure heaters for maximum cycle heat recovery when operating on the start-up system.

Also part of the start-up system are pressure reducing Valves -94 disposed in the conduits 48 between the roof tubes and the primary superheater, stop valves 96 and 98 in the bypass take-off and return lines, and stop valves 97 in the main flow line 54 between the points of connection of the bypass take-off and return lines.

ln operation, the boiler is started up by first directing the Working fluid flow through the circuit to the primary superheater outlet, and diverting the flow into the bypass system 80. For this purpose, stop valve 97 is closed and stop valve 96 is open. Pressure reducing valves 94 are in operation maintaining full furnace pressure, and a reduced pressure in the primary superheater for maximum heat pickup, and downstream thereof. The burners for the generator are lighted, and when steam becomes available in the flash tank, part is diverted to heat recovery sections of the cycle, and part returned via line 86 to the main flow line for warming and initial rolling of the high pressure turbine.

As heating in the generator continues, enough steam is generated to apply a load to the turbine, and loading progresses. During start-up, the flow through the generator is at about 30% of full load flow rate, 30% being that conventionally used to assure adequate cooling of the heating surfaces in the generator during start-up. At about load, the full 30% flow is diverted into the bypass system, about 15% being returned to the main flow line via line 86, about 15% being diverted to heat recovery portions of the cycle. Further loading is accomplished by opening main line stop valve 97 and closing the bypass line stop valve 96 upstream of the flash tank. Eventually the full flow is transmitted to the high pressure turbine corresponding to a 30% load on the turbine. Further loading of the turbine is accomplished by increasing the firing rate and pumping rate in the generator in response to load demand.

As is well known the vapor flow from the flash tank returned to the generator main flow path or line is saturated steam. This is further heated and superheated in the finishing superheater, or surface downstream of the bypass system, so that the turbine is assured a steam flow of desired enthalpy. During closing of the bypass line 86, after 15 load, there continues to be transmitted to the turbine through main line stop valve 98 a high enthalpy flow by virtue of the higher heat pick-up achieved in the primary superheater, to the extent that with permissible firing rate, 26%, no enthalpy drop is experienced at the turbine throttle inlet during change-over from bypass flow to main line fiow. Admittedly, a `function of heat transfer is the amount of surface involved. For purposes of this invention it is assumed that the amounts of surface provided for vapor generation and superheating are appropriate for normal operation of the generator. The present invention overcomes a problem experienced in conventional units without the addition of surface or increase in size of the generator, using surfaces sized in accordance with conventional design criteria.

During the initial start-up period up to turbine synchronization, the finishing superheater has no flow 0r minimal flow in it. For purposes of this invention, both the pendent superheater 28 and the finishing superheater section 30 will be considered the finishing superheater. The absence of significant flow, particularly in the pendent superheater section 28, requires that the gas temperature be less than 1200 over the surfaces. For this purpose, a temperature probe 102 is disposed immediately below the pendent superheater section 28 connected to controls for the burners to limit the firing rate and gas temperature at this point. It is Within the scope of this invention to use other types of surfaces than the pendent type surface at this point, for instance platen superheaters or a partial division Wall. In each instance, the temperature probe would be disposed with respect to the surfaces to assure against overheating.

An unexpected advantage discovered in accordance with the invention is that the desired enthalpy level can be achieved at 15% turbine load upstream of the bypass circuit at the permissible firing rate of approximately 26%, which does not jeopardize the finishing superheater surfaces through overheating. Conventionally, higher tiring rates are needed to reduce the shock problem experienced in most units, which may result in overheating the finishing superheater metals.

Since full or partially full load operation is that for which the boiler is designed, the most efficient arrangement of surfaces is that which provides efficiency at these loads. This is the arrangement of surfaces shown with the primary superheater in a low gas temperature zone, freeing the higher gas temperature zones for the finishing superheater sections. In the lower gas temperature zone, an efiicient heat transfer is achieved despite the lower temperature. Positioning the finishing superheater sections in higher gas temperature Zones assures an adequate temperature level at the turbine inlet during all loads of the boiler, including start-up.

Athough the invention has been described with reference to a specific embodiment, other embodiments and variations within the Scope of the following claims will be apparent to those skilled in the art.

What is claimed is:

l. The method of starting up a once-through vapor generator-turbine unit comprising the steps of establishing a flow of about 30% of full load flow in the generator;

bypassing said flow prior to final heating thereof;

reducing the pressure of the flow and reheating the reduced pressure ow, both steps occurring prior to bypassing the ow;

separating the bypassed flow into a vapor stream and a liquid stream and finally heating said vapor stream;

the reheating of the reduced pressure fiow being carried out in heat exchange with a lower temperature gas than that used in the final heating of the vapor stream;

the amount of pressure reduction prior to reheating being sufficient to achieve a saturated vapor condition of the flow following reheating at a firing rate of approximately 26% of full load firing rate and at about turbine load, such that any enthalpy level which will assure matching of said vapor stream with said turbine unit will be reached upstream of the bypassed flow.

2. The method of starting up a once-through generatorturbine unit of the type comprising a main ow path including vapor generating surface, primary superheater surface, and finishing superheater surfaces for final heating of the vapor, the surfaces being arranged for series ow therethrough, comprising the steps of establishing a fiow of about 30% of full load ow through the generating surfaces;

bypassing said flow from the main flow path prior to final heating thereof but subsequent to flow in said primary superheater surface;

reducing the pressure of the flow and reheating the reduced pressure flow in said primary superheater surface prior to bypassing the fiow;

separating the bypass flow into a vapor stream and a liquid stream and finally heating said vapor stream in said finishing superheater surface;

the reheating of the reduced pressure flow being carried out in heat exchange with a lower temperature gas than that used in the final heating of the vapor stream;

the amount of pressure reduction prior to reheating being sufiicient to achieve a saturated vapor condition of the flow following reaheating at a firsting rate of approximately of full load firing rate and at about 15% turbine load that an enthalpy level which Will assure matching of said vapor stream will be rotated upstream of the bypassed flow prior to changeover to said main flow path.

3. The method of claim 2 wherein the firing rate is sufficiently low to hold gas temperatures across th; finishing superheater at values compatible with the cooling steam flow available internally in the surface, thereby avoiding overheating of the nishing superheater metals.

4. A once-through vapor generator comprising a generator enclosure;

a main flow path including vapor generating surface, primary and finishing superheater surfaces within the enclosure;

burner means heating said enclosure to define a radiant heat transfer area and a convection heat transfer area, the primary superheater surface being disposed in a relatively cold gas temperature zone of the convection heat transfer area freeing higher gas temperature zones including in part radiant heat transfer area for said finishing superheater surface;

a start-up bypass flow path connected to said main flow path at a first point intermediate said primary and finishing superheater surface, the bypass flow path including vapor liquid separating means and conduit means connected to the separating means and main flow path at a second point between the first point and finishing superheater surface to return a vapor stream to said main ow path;

stop valve means between said first and second points;

and

pressure reducing valve means in the main ow path upstream of the primary superheater whereby during start-up the heat transfer rate into said primary superheater surfaces is maximized such that an enthalpy level which will assure matching of steam from said separating means will be reached upstream of said bypass fiow path prior to changeover to said main flow path.

5. A generator according to claim 3 wherein said finishing superheater includes pendent superheater surfaces disposed in the radiant heat transfer area of the generator, and additional surface immediately downstream of the pendent superheater surface with respect to the direction of gas fiow in a convection heat transfer but relatively high gas temperature area of the generator.

6. A generator according to claim 4 wherein the generating and superheating surfaces are sized for normal operating conditions of the generator.

'7. A once-through vapor generator comprising enclosure means defining zones of high temperature,

intermediate temperature and relatively low temperature;

heat transfer vapor generating surface to generate vapor during normal operation;

additional heat transfer superheating surface to superheat the vapor, said additional heat transfer superheating surface including a primary surface and a finishing surface for final heating of the vapor;

the primary surface including an outlet and being disposed in a relatively cold gas temperature zone, freeing higher gas temperature zones for Said finishing surface;

pressure reducing means intermediate the vapor generating surface and said primary surface so that a reduced pressure fiow is maintained in the latter during start-up; bypass flow means adapted to receive the primary surface outlet flow during start-up, said bypass flow means including separation means to provide during start-up a liquid stream and a Separate vapor stream;

means to transmit said vapor stream to said finishing surface;

the pressure reducing means being sized to reduce the pressure to that required to obtain during start-up, at a heating rate of less than saturated vapor at the primary surface outlet at about 15% load, such that an enthalpy level which will assure matching of steam from said separation means will be reached upstream of said bypass flow means prior to changeover to said heat transfer generating surface and said heat transfer superheating surface.

References Cited UNITED STATES PATENTS (35 KENNETH W. SPRAGUE, Primary Examiner UNITED STATES PATENT oFFICE CERTIFICATE OF CORRECTION Patent No. 3 ,472 ,208 October I4 1969 Walter P. Gorzegno et al.

It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column l, line 32, after "generator" insert to Column 5, line 38, "reaheating" should read reheating same line 38 "firsting" should read firing line 40 after "load" insert such line 42, "rotated" should read reached Signed and sealed this 21st day of April 1970.

(SEAL) Attest:

WILLIAM E. SCHUYLER, JR

Commissioner of Patents Edward M. Fletcher, Ir.

Attesting Officer 

